Many processes in the semiconductor industry require a reliable source of process gases for a wide variety of applications. Often these gases are stored in cylinders or vessels and then delivered to the process under controlled conditions from the cylinder. The semiconductor manufacturing industry, for example, uses a number of hazardous specialty gases such as phosphine (PH3), arsine (AsH3), and boron trifluoride (BF3) for doping, etching, and thin-film deposition. These gases pose significant safety and environmental challenges due to their high toxicity and pyrophoricity (spontaneous flammability in air). In addition to the toxicity factor, many of these gases are compressed and liquefied for storage in cylinders under high pressure. Storage of toxic gases under high pressure in metal cylinders is often unacceptable because of the possibility of developing a leak or catastrophic rupture of the cylinder.
One recent approach to storage and delivery of Lewis acid and Lewis base gases (e.g., PH3, AsH3, and BF3) resides in the complex of the Lewis base or Lewis acid in a reactive liquid of opposite Lewis character, e.g., an ionic liquid (e.g., a salt of alkylphosphonium or alkylammonium) of opposite Lewis character. Such liquid adduct complexes provide a safe, low pressure method of storage, transporting and handling highly toxic and volatile compounds.
The following reference illustrates a delivery apparatus for Lewis basic and acidic gases from reactive liquids and proposed mechanisms for the formation of Lewis complexes of Lewis gases with reactive liquids and for recovering the gases from the reactive liquids and delivering the respective gases to the onsite facility.
U.S. Pat. No. 7,172,646 (the subject matter of which is incorporated by reference) discloses a process for storing Lewis base and Lewis acidic gases in a nonvolatile, reactive liquid having opposing Lewis acidity or Lewis basicity. Preferred processes employ the storage and delivery of arsine, phosphine and BF3 in an ionic liquid.
Complexed gas technology presently utilizes a volume of bulk reactive liquid contained in a cylindrical vessel. The vessel may be oriented horizontally or vertically during use. The liquid is prevented from exiting the vessel by a gas/liquid separator barrier device. The separator may, for example, contain a thin, microporous membrane designed to allow passage of gas while preventing liquid passage out of the vessel. This apparatus suffers from operational limitations such as: a potential for minute liquid leakage through the microporous phase barrier to the outside, a potential for membrane rupture leading to substantial liquid release to the outside, a requirement to keep the vent positioned in the gas space of the vessel during use regardless of vessel orientation, a potential for increased flow restriction through the membranous phase barrier due to liquid or solid deposits on the membrane, a potential for flow and pressure fluctuations during gas delivery due to sub-surface hydrodynamic effects such as bubbling and convective liquid flow in the bulk liquid volume, and a relatively small ratio of free surface to volume in the bulk liquid leading to a limited interfacial mass transfer rate leading to (1) a limited rate of gas complexation, (2) a limited rate of gas fragmentation and (3) incomplete fragmentation or delivery of gas product.